Using profanity and pushing another student used to mean a visit to the principal's office. But these days, such lesser violations account for most of the 3.3 million public school suspensions each year. At work here are the nation's "zero-tolerance" discipline codes, and black kids are disproportionately feeling the pain. One case in North Carolina is heading to the state Supreme Court, where lawyers plan to argue that students' constitutional rights are being violated.

Poor black students are suspended at three times the rate of whites, a disparity not fully explained by differences in income or behavior. …

On March 8, the education secretary, Arne Duncan, lamented “schools that seem to suspend and discipline only young African-American boys” as he pledged stronger efforts to ensure racial equality in schooling.

A growing body of research, scholars say, suggests that heavy use of suspensions does less to pacify schools than to push already troubled students toward academic failure and dropping out — and sometimes into what critics have called the “school-to-prison pipeline.”

A rising number of districts are already reversing course and trying new approaches, including behavioral counseling and mediation, to reduce conflict and create safer, quieter schools while ejecting only the worst offenders.

“These students were treated like criminals and abandoned by the school system for doing something that students have done forever — fighting in the schoolyard,” said Erwin Byrd, a lawyer with Legal Aid of North Carolina, which brought the suit with lawyers from the Duke University School of Law. The school district says it must retain discretion over punishments.

Some 15 percent of the nation’s black students in grades K-12 are suspended at least briefly each year, compared with 4.8 percent of white students, according to federal data from 2006, the latest available. Expulsions are meted out to one in 200 black students versus one in 1,000 white students.